As an optical pickup to be used to write and read information signals on or from opto-magnetic disk, phase-transition type optical disk, or other optical media, a high-power semiconductor laser device capable of delivering stable and high optical output power without producing interference between the light reflected from optical medium and the light introduced thereon, has ben strongly desired.
However, since the high-power single mode laser device is inevitably unstable particularly when it is in a read condition delivering a minute optical power output, and the light reflected from the optical medium is inputted therein. In order to improve this unstable condition, the semiconductor laser device has been conventionally driven by current on which high-frequency current of an order of several hundred mega Hertz is superposed. By this, a multi-mode oscillation spectrum can be obtained, and the laser stability against the reflected light can be substantially improved.
A schematic construction of such conventional semiconductor laser device is shown in FIG. 2 wherein 1 is a semiconductor laser chip, 2 is a package stem in which said semiconductor laser chip is mounted, 3 is a glass window delivering the optical output of said semiconductor laser device, 4 is a printed circuit board on which a high-frequency circuit is disposed, 5 is an electronic elements constituting said high-frequency circuit, and 6 is a package in which said high-frequency circuit is provided.
In accordance to the development of compact optical disk, the development of more compact optical pickup has been strongly desired, and for this, an optical unit in which the components of optical pickup are integrated, has been proposed.
An example of the proposed unit is shown in FIG. 3 wherein 7 is a silicon substrate, 8 is a mirror inclined at an angle of 45.degree., 9 is a photo-detector detecting optical signals, 10 is a frame made of resin, 11 is a comb-frame, 12 is a hologram element, 13 is an outputted laser light, and 14 is a light modulated at the surface of optical disk and reflected to the laser device.
The construction of the proposed unit is explained below by referring FIG. 3 wherein semiconductor laser chip 1 is disposed on silicon substrate 7, and outgoing laser light 13 is directed along a direction vertical to said substrate after it is reflected by said mirror inclined at an angle of 45.degree..
Said outputted laser light 13 is introduced on an objective lens through a collimator lens provided at a side of said pickup, before said light 13 is focused on an optical disk. The light is modulated thereon, and is reflected from the surface of said optical disk, and is returned to said optical unit. At this time, a part of the modulated light is refracted by hologram element 12 and the refracted light is detected by photo-detector 9 fabricated on silicon substrate 7.
Although FIG. 3 shows a typical optical unit in which various optical functions are integrated, the method to superpose the high-frequency signal on the injection current of semiconductor laser device had not been well refined. Therefore, when this optical unit is utilized as an optical pickup, and when the light reflected from an optical disk is detected by a photo-detector, the output of the photo-detector is unstable, and readout signals containing a large amount of noise are generated.
Thus, similar to the construction of conventional semiconductor laser device shown in FIG. 2, a device construction wherein the high-frequency circuit elements are directly disposed on the terminals of semiconductor laser device, has been considered. A typical device construction of such is shown in FIGS. 4(a) and 4(b) wherein the high-frequency signal is applied to the semiconductor laser through the terminals of comb-frame 11.
In this case, in order to prevent the radiation of high-frequency signal from the electrode terminals, the conventional semiconductor laser device including its terminals shown in FIG. 2 have to be sealed in metal package stem 2, and the high frequency circuit has to be sealed in metal package 6, and these two metal packages have to be bonded and grounded.
Furthermore, with the device construction shown in FIGS. 4(a) and 4(b), package stem 2 of semiconductor laser device is not completely shielded, and therefore, the high-frequency radiation from the termi-nals and its interaction with the other circuits are inevitable. Moreover, because of the excessive device thickness especially along its optical axis, most of the advantages of the optical device integration would be lost.